Bill Gates says a pandemic flu could kill 30 million people — these are the deadly diseases that scare experts the most

A pandemic disease could be the greatest threat humanity faces right now.

The WHO and CDC each keep lists of diseases that they consider urgent priorities for more research, but the next pandemic could be caused by a disease people have never seen before.

For now, though, these are the six diseases that worry experts the most.

For scientists that study disease, one unnerving fact is always present: somewhere on the planet, there's an organism or organisms - likely bacteria or a virus - that could kill tens of millions if it started to spread among people.